As is well known, a superconductive magnet has a main coil of superconductive material which is located in a cryostat and is cooled to a temperature near absolute zero by liquid helium. By superconduction, the main coil creates a high intensity, primary magnetic field into which, for example, the body being imaged is positioned. A plurality of superconductive shim coils are shaped and located relative to the main coil to correct for aberrations in different field gradients in the primary magnetic field.
The operation of a superconductive magnet occurs in three phases. First is a charging or energization phase during which a power supply external to the cryostat increases the current until it reaches the desired operating level. Second is an operating or normal phase during which current carried by a superconductive coil generates the desired magnetic field. In a "driven" magnet, the power supply remains connected to the main coil whereas in a "persistent" magnet, a persistent switch heater is de-energized allowing the power supply to be disconnected. A persistent current continues to flow through the coil and a superconductor connected across the coil. Third is a discharging or de-energization phase during which current is slowly decreased through an external circuit or power supply. Shim coils are also operated with the three phases and each shim coil is selectively energized during the first phase to develop the desired correction gradient.
Quenching is the phenomenon of a superconductor switching or transiting from a superconductive state, in which it has virtually no resistance, to a normal state in which it has some resistance. In large magnets such as required for magnetic resonance imaging, the magnetic field stores a tremendous amount of energy. If quenching occurs, such energy must be dissipated in a controlled manner to prevent damage to the magnet and preferably to prevent the coolant, liquid helium, from boiling off or vaporizing in a violent manner. While quenching can occur during any phase of operation, it more often occurs during the start-up or energization phase.
The problem of safe quenching is known in the prior art and different solutions have been provided particularly for protecting the main coil. One solution involves connecting an external resistor across the main coil of a driven magnet. When quenching begins, the power supply is disconnected forcing current through the resistor. The bulk of the magnetic field energy is thereby resistively dissipated outside of the magnet without causing physical damage or coolant boil-off. Another solution for a persistent magnet involves the recognition that the magnetic field energy can be safely dissipated in the mass of the magnet itself and by allowing coolant boil-off. In one such solution, the main coil is divided into a plurality of segments each having a resistive heater electrically connected in shunt therewith. Each heater is also physically shaped and placed adjacent to a different segment. When quenching occurs, it normally starts in a small section of the coil and spreads. The change in resistance of the section from zero to a higher value forces current through the shunt resistor which then heats another segment causing it to quench. This action spreads until all of the superconductive coils have switched to a resistive state. The energy is dissipated by resistive heating of all the coils and resistors which heating raises the temperature of the magnet without causing any localized fusing, melting, excess thermally induced stress or other damage. Some heat also is transferred into the liquid helium causing it to boil off. After quenching has occurred in such a controlled manner, the magnet can be started by refilling the cryostat with coolant and by re-energizing the magnet.
However, the solution to the problem of protecting shim coils is not so simple. Shim coils are designed to carry less current than the main coil and the conductors are accordingly smaller. Further, shim coils have fewer turns than the main coil and are mutually inductively coupled with the main coil so that small changes in current in the main coil induce much greater changes in the shim coil current. The result may be that quenching in the main coil, which reduces current therein, can so increase the current in the shim coils to quench them and other conductors causing them to melt or fuse. The large induced currents in the shim coils can also produce unacceptably large forces on the shim coils and associated structure. The invention overcomes this problem, in the manner discussed below.